The present invention relates to a portable catalytic oxidizers for use in the destruction of hydrocarbon vapors in order to purify an air stream and prevent pollution. Typically the most common application of such catalytic oxidizers is in the destruction of vapors extracted insitu from soils that have been contaminated with hydrocarbons, typically by a hydrocarbon leak from a storage tank. Such applications normally involve the clean-up of vapors contained in an air stream generated during clean-up of hydrocarbon spills where the contaminated air stream is near ambient temperatures. The hydrocarbon content of the air stream is diluted if necessary to keep it below the lower explosion limit (LEL). In prior art devices directed to such applications, the catalyst employed for catalytic combustion usually employs platinum group metals, typically comprised of finely divided platinum and/or (palladium) deposited on a high surface area alumina substrate, which in turn is deposited on a honeycomb support structure of stainless steel or ceramic. The advantage of a catalytic combustion process is that it can be operated at a much lower temperature and lower hydrocarbon concentration than is possible with ordinary flame combustion. Many applications involve very low concentrations of hydrocarbon where catalytic combustion is efficient, while flame type combustion is not practical.
The catalytic oxidizers used in hydrocarbon spill cleanup, such as the present invention, normally operate with inlet air hydrocarbon vapor mixture concentrations less than 25% of the lower explosive limit (LEL). The air hydrocarbon vapor mixture source may have much higher concentration, frequently in the explosive range. In such cases, the preferred mode of operation is to provide controlled dilution of the source stream with air, so as to maintain the inlet concentration to the catalytic unit within acceptable limits. If the inlet air hydrocarbon vapor mixture concentration exceeds the lower explosive limit (LEL), an ignition might occur within the catalytic unit. Excess pressures resulting from such ignition of high air hydrocarbon vapor mixture concentration have typically been compensated for in prior art catalytic converters by providing a vessel having substantial wall thickness and mass to contain such overpressures. Other prior art devices employ blast vents and doors to relieve excess pressures.
Accordingly, it is an object of the present invention to provide a catalytic oxidizer that is portable, compact and minimizes the overpressure that can develop due to ignition of air hydrocarbon vapor mixture mixtures within it.
Applicant is aware of several forms of prior art catalytic oxidizers, commonly called catalytic incinerators. These incinerators are commonly assembled by piping the basic elements together and enclosing the heated portion including the catalyst in an insulating jacket. Such incinerators are generally of the form including a catalyst chamber, a heater, a heat exchanger of fixed-tube-sheet construction employing a plurality of heat exchanger tubes, thermocouple sensors, piping and automatic controls. One such device is covered by U.S. Pat. No. 3,898,040 to F. Tabak which discloses a compact integrated design for an incinerator that combines a burner with a catalytic operation. While the Tabak incinerator differs markedly in structure, it combines heat recovery and catalytic combustion in a singular shell. Other catalytic fume incinerators are of an integrated design but of different heat exchanger configuration from the applicant's current invention are disclosed in U.S. Pat. No. 2,861,873 to Worn and U.S. Pat. No. 4,770,857 to Ludwig. Further patent to U.S. Pat. No. 3,466,152 to Yamamoto et. al., discloses a synthesizing reactor disclosing a vessel having a substantial weight in thickness employing high pressure synthesis including a plurality of spaced catalyst chambers, each having an in and an outlet, and a heat exchanger is disposed in a separate chamber employing flow tubes and baffles to facilitate heat exchange between unreacted gas and reacted gas.
Contrary to Yamamoto and the other prior art, the current invention has identified a problem hitherto not recognized by the prior art which is that catalytic oxidizers used for the clean-up of hydrocarbon spill operations require portability, with explosion protection adapted to be carried to various sites or remote locations. The current invention has solved this problem by combining an counterflow, spiral-path, heat exchanger coaxially wrapped around a central catalyst core employing an electric heater, a safety relief device for relief of excess pressure and an insulated outer shell into a compact integrated assembly thus minimizing piping, insulation and generally the weight of materials needed to assemble the unit by utilizing the improved heat transfer of the spiral path counter flow heat exchanger configuration.
It is therefore desirable to provide an improved portable and highly efficient, lightweight, compact catalytic oxidizer for the destruction of hydrocarbon vapors for purification of an air stream and to provide a method of oxidizing hydrocarbon vapors by utilization of a catalytic oxidizer apparatus having a coaxial heat exchanger which is simple to operate, highly efficient, explosion resistant and is portable.